In the field of waveguide filters it is known to use a housing comprising a bottom portion, a top portion and side portions connecting the bottom portion and the top portion. The housing comprises an input port (also called an iris) for receiving an input signal and an output port for transmitting an output signal. The filter receives the input signal and filters the input signal from certain frequencies and forwards the filtered signal as the output signal to a transmitting unit or transmits the output signal itself. The housing is designed dependent on the filter type, for example a band-pass filter or a low-pass filter. However, common for all filters are that the housing comprises one or more cavities which together with corresponding trimming means is designed for filtering the input signal. The trimming means are in the form of one or a number of movable bodies corresponding to the number of cavities. U.S. Pat. No. 5,349,316 teaches the use of movable bodies in the form of trimming plungers or trimming screw.
U.S. Pat. No. 4,041,421 teaches the use of movable bodies in the form of screws being rotatably mounted in the top portion and each screw protrude into one of said cavities. It is long known that the volume of a trimming means body in the cavity is essential for the filter behaviour. Therefore, when trimming the filter a predetermined input signal is fed to the filter whereafter the output signal is measured while the screws are screwed into or out from the cavities until a predetermined and desired output signal pattern is achieved.
The need for the use of trimming means is to adjust the operating frequency of the filter to meet a desired frequency characteristic. The trimming means is either screwed into or out from the cavity in order to change the capacitance of the waveguide filter. The trimming is necessary because the position of the trimming means in the cavity and the material of both the trimming means and the waveguide filter affects the capacitance. Furthermore, lack of dimension accuracy giving cavities non-predictable sizes also gives a need for adjusting the trimming means in order to compensate for the differences in order to get a number of filters to perform identically to another for the same filtering purpose. Hence, every prior art waveguide filter have to be trimmed after assembly which is a time consuming process step and therefore also a costly part of the manufacturing.
One problem with using screws is that the size of the housing decreases with the increase in frequency and for frequencies above 20 GHz the housing is so small that the screws, the holes for the screws and the threads for the screws become so small that the process of manufacturing the same is difficult and costly. In addition to this problem, the small sized screws are hard to handle because of their smallness and the trimming of the filter becomes difficult and costly.
Thus, there exists a need for an improved waveguide filter being easier and cheaper to manufacture.